1. Field of the Invention
This invention relates to a control system for an engine provided with a mechanical supercharger which is driven by the engine output power.
2. Description of the Prior Art
There has been known an engine with a mechanical supercharger which is driven by the output power of the engine and is made inoperative by means of an electromagnetic clutch or the like when the engine is started or the vehicle is backed. See Japanese Unexamined Patent Publication Nos. 61(1986)-19932 and 60(1985)-240838, and the like.
Further it has been known to make the mechanical supercharger inoperative and to feed intake air through a bypass passage which bypasses the mechanical supercharger in a low engine speed range lower than a predetermined value in order to reduce the driving loss and improve the fuel economy since the power increase requirement is relatively low when the engine speed is low and the engine load is light.
On the other hand, when the mechanical supercharger begins to be driven in a high engine speed range, an immense engaging shock is generated when the mechanical supercharger is operatively connected to the engine output shaft, which adversely affect the reliability of the clutches, the mechanical supercharger the driving belt and the like. Accordingly, it is preferred that the mechanical supercharger begins to be driven before the engine speed increases to the high engine speed range.
As the mechanical superchargers, there have been known various types such as a root blower type, a screw type and the like.
When such a mechanical supercharger is operatively connected to or disconnected from the engine output shaft at a predetermined engine speed, torque shock is generated due to increase and reduction of load on the engine, and it seems that the lighter the engine load is, the larger the torque shock is.
In FIG. 5, curve A shows the relation between the engine load and the torque which engine produces when the mechanical supercharger engages while curve B shows the same when the mechanical supercharger disengages. The curves A and B cross at a predetermined engine load, and in the engine load range lighter than the predetermined value, the engine produces higher torque when the supercharger do not engages than when the supercharger engages. As can be understood from the comparison of the curves A and B, as the engine load at which the mechanical supercharger engages and begins to operate becomes lighter, the reduction of the torque becomes larger in the engine load range lighter than the predetermined value.
Since the engaging shock is immense when the mechanical supercharger engages at a high engine speed as described above, and the driver experience more shock for a given torque fluctuation as the engine load reduces, it has been a great demand for suppression of the torque shock which is generated when the mechanical supercharger begins to operate.